Authorities were searching Sunday for two men suspected of shooting and wounding a police officer in Ferguson, the St. Louis suburb where there have been angry protests since a white officer fatally shot an unarmed 18-year-old black man last month.

Missouri’s governor on Monday ordered the National Guard to a St. Louis suburb convulsed by protests over the fatal shooting of an unarmed black teen, after a night in which police used tear gas to clear protesters off the streets well ahead of a curfew.

After an unarmed black teenager was shot by a Ferguson, Missouri, police officer, the city north of downtown St. Louis erupted into violent protests. Here’s a look at the key elements of the shooting and the unrest that followed.